Camber Girder

Camber in girders is an intentional upward curvature introduced during the fabrication of a girder. It compensates for anticipated deflections due to dead loads, live loads, and other effects, ensuring the structure achieves a desired profile when loaded. Standards and practices for cambering depend on design specifications, construction codes, and regional standards. Below are key considerations and typical standards related to cambered girders:

1. Camber Standards and Guidelines

• AASHTO (American Association of State Highway and Transportation Officials):

  • Provides guidelines for bridge girders, including camber requirements to offset deflections caused by dead load and live load.
  • Camber is calculated based on material properties, girder span, and load factors.

• ACI (American Concrete Institute):

  • For reinforced and prestressed concrete girders, ACI 318 recommends cambering to counteract long-term deflection from creep and shrinkage.

• AWS D1.1/D1.5 (American Welding Society):

  • Outlines practices for fabricating cambered steel girders, including heat cambering or mechanical means to achieve the curvature.

• Eurocodes (EN 1993 & EN 1994):

  • European standards provide formulas for calculating deflections and setting camber for steel and composite girders in bridges and buildings.

• Indian Standards (IS 800, IS 456):

  • Include recommendations for camber in steel and concrete girders based on anticipated deflections and prestress losses.

2. Methods for Setting Camber

• Mechanical Cambering:

  • Girders are bent into shape using presses or rollers during fabrication.

• Heat Cambering:

  • Localized heating is applied to induce curvature through thermal expansion and contraction.

• Precast Concrete Girders:

  • Camber is introduced by adjusting prestressing tendons.

• Field Adjustments:

  • Minor camber can be introduced during erection by temporary supports or jacks.

3. Design Considerations

• Camber Calculation:

  • Typically based on:
    • Dead load deflection.
    • Prestress losses (for prestressed girders).
    • Live load effects (if significant).
    • Long-term deflections (creep and shrinkage in concrete).

• Tolerances:

  • Tolerances for camber are specified to ensure compatibility during construction.
  • Example: Steel girders often have a tolerance of ±5 mm or as specified in the design.

• Profile Coordination:

  • Designers must coordinate camber profiles with roadways or floor slabs to ensure proper alignment.

4. Standard Camber Values

While there is no universal “standard” camber value, typical camber is determined using the following formula for steel and concrete girders:

$$\text{Camber}=\text{Deflection due to Dead Load}+\text{Allowances for Other Effects}$$

For concrete girders, the values are more variable due to creep and shrinkage. For steel girders, the calculated camber is generally close to:

• For bridges: Approximately $1/800$ to $1/1000$ of the span length.
• For building girders: Typically $1/240$ to $1/300$ of the span length.

Key References

1. AASHTO LRFD Bridge Design Specifications.
2. AWS D1.1/D1.5 Structural Welding Codes.
3. ACI 318-19: Building Code Requirements for Structural Concrete.
4. IS 800: General Construction in Steel.
5. Eurocode 3: Design of Steel Structures.